1. Field
The disclosed concept pertains generally to input circuits and, more particularly, to input circuits for alternating current signals. The disclosed concept also pertains to electrical apparatus, such as motor starters.
2. Background Information
Capacitive coupling occurs when conductors, such as I/O (input/output) lines used to carry signals (e.g., without limitation, signals for motor overloads, such as an input signal to reset a motor after the occurrence of a trip), are in close proximity to other conductors that carry power. These conductors may all be coupled closely together in the same wire tray or even the same cable pack.
FIG. 1 shows a typical configuration including the potential for capacitive coupling. A remote switch S1 2 can be 100 to several 1000 feet away from a motor starter 4. In this example, a 120 VAC hot line 6 is energized at all times. The capacitor C1 8 is not real, but represents the fact that two elongated conductors 10 travel a relatively long distance in a cable pack (not shown), in order that the conductors are physically side by side and therefore act as two plates of a capacitor that are coupled relatively tightly together. The longer the conductors 10 travel together the greater the capacitance. One plate of this capacitor has the 120 VAC voltage applied at all times. The other plate is pulled to ground through resistor R1 12 when switch S1 2 is open. Circuitry (not shown) internal to the motor starter 4 monitors the voltage across R1 12 to determine if a valid input signal is present. This particular example has a threshold set at a predetermined value, such as 5 VDC. Any signal above 5 VDC would be considered to be a single valid logic high and any signal below 5 VDC would be a valid logic low. With switch S1 2 open, the voltage on R1 12 is a function of the magnitude of the capacitance of capacitor C1 8 and the magnitude of the resistance of resistor R1 12. These two components form a high-pass filter whose output voltage, Vout, is given by Equation 1.Vout=2πFREQ(Vin)(C1)(R1)/((2πFREQ(C1)(R1))2+1)0.5  (Eq. 1)wherein:                Vin is AC input voltage (e.g., without limitation, 120 VAC); and        FREQ is frequency (e.g., without limitation, 60 Hz) of the AC input voltage.        
Plugging in appropriate values gives the results shown in Table 1:
TABLE 1Vout5 VDCC1R1FREQVinVoutPeakvalid(F)(Ω)(Hz)(VACRMS)(VACRMS)(V)logic high5.00E−0810000060120106.0149.9No5.00E−091000006012022.231.4No5.00E−10100000601202.33.2Yes5.00E−0510060120106.0149.9No5.00E−061006012022.231.4No5.00E−07100601202.33.2Yes
If the motor starter input impedance is relatively high (e.g., R1=100 KΩ), then the cabling can only have the capacitance of C1 be about 0.5 nF (5.00E-10 F) before the threshold is exceeded with S1 2 open. The capacitance of C1 8 being greater than 0.5 nF gives an invalid logic high. When the input impedance is changed to 100 Ω, the capacitance of C1 8 being greater than 0.5 μF (5.00E-07 F) gives an invalid logic high. Cabling capacitance can become 1000 times greater before false readings can occur. Larger capacitance handling would allow much longer cable lengths.
Hence, a valid signal on an input line can have capacitive coupling issues due to relatively long distance runs or due to a relatively high voltage in close proximity to the input line.
It is known to employ a synchronous input circuit that turns on a load bank for approximately 4 mS at every zero-crossing. The load bank is turned on 2 mS before each zero-crossing and held on until 2 mS after the zero-crossing. This requires knowing exactly when the zero-crossings occur. The voltage across the load bank is relatively small during this time interval.
There is room for improvement in input apparatus.
There is also room for improvement in electrical apparatus, such as motor starters.